In a communication system for transmitting/receiving signals over e.g., a cable, in particular a system for effecting full-duplex simultaneous bi-directional communication, over the same cable, the performance of a transceiver, in particular that of a receiver circuit, is deteriorated by an echo, which is a reflected signal of the signal transmitted from the same transceiver device, produced by impedance mismatching of a transmission line, or the crosstalk ascribable to spatial capacitance coupling from the other cable. Hence, a need exists for a canceller function for canceling the effect of echo/crosstalk in the received signal to reproduce a true received signal.
In a conventional canceller function, a continuous time analog signal waveform in general is once converted by an analog-to-digital converter (termed also as AD converter) to a discrete-time digital signal, obtained on quantizing the amplitude, and the effect of the echo and the cross-talk is canceled by digital signal processing employing a digital adaptive filter (see, for example, the Patent Document 1 referred to below). It is noted that the period of the discrete time corresponds to a reciprocal of the conversion frequency of the AD converter, while the unit of quantization is prescribed by the resolution of the AD converter. As an example of the communication system, forming the background art of the present invention, there is shown in FIG. 11 a gigabit Ethernet (trademark) (Gigabit Ethernet (registered trademark)), having a hybrid and a canceller and configured for performing full-duplex bidirectional operations (see the Patent Document 1 referred to below). In FIG. 11, the communication between a computer 14 and a hub 12 is carried out using a UTP (unshielded twisted pair) of a wire or a cable (formed of copper) 18, operating at 250 Mb/sec, and eight transceivers 20, located at both ends of the UTP 18. A gigabit media independent interface (GMII) 28 receives data in a byte form, at e.g., 125 MHz, to output the data to a physical coding sub-layer (PCS) 30 forming a physical layer. The PCS 30 encodes bit data from the GMII 28 into a 5-level pulse amplitude modulated (PAM) signal. The five signal levels are −2, −1, 0, +1, +2. Both ends of each UTP 18 are provided with the hybrids 26 because these are needed in the full-duplex bidirectional operations. The hybrid 26 controls access to a communication line 16, thereby allowing for full-duplex bidirectional operation between the transceivers 20 at each end of a communication line. The hybrid also functions to isolate a transmitter circuit (TX) 22 and a receiver circuit (RX) 24 associated with the transceiver from each other. This Patent Document 1 discloses the structure of a transceiver 20 in which signals form the hybrids 26 are converted by an AD converter, not shown, to a digital signal at 125 MHz, and in which an output signal of the AD converter is received by in an FIFO (first-in first-out) storage circuit, not shown, and stored therein in the first-in first-out system, so as to be then output to an adaptive filter (feedforward equalizer (FFE)), not shown. The FFE performs adaptive filtering based on the LMS (least mean square) algorithm to correct the distortion by channel equalization and inter-symbol interference (ISI) cancellation. The sum of an output of the FFE, an output of an echo canceller, not shown, and an output of a NEXT cancellation system, is supplied to a detector. There is provided a timing recovery circuit, supplied with an error formed by an output of the detector and an input to the detector, and with the detector output, to send clocks to the FIFO and to the AD converter.
There is also known an echo canceller, as an echo canceller of the ISDN (Integrated Service Digital Network), configured for operating at approximately four times the data rate, in order to cover a signal band of the modified biphase code (see for example the Non-Patent Document 2 referred to below).
[Patent Document 1]
U.S. Pat. No. 6,236,645 (FIGS. 2 and 6)
[Non-Patent Document 1]
AKIRA KANEMASA, AKIHIKO SYGIYAMA, SHIN'ICHI KOIKE, and TETSU KOYAMA, “An ISDN Subscriber Loop Transmission System Based on Echo Cancellation Technique”, IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, pp. 1359-1366, VOL. SAC-4, NO. 8, NOVEMBER 1986
[Non-Patent Document 2]
KURT H. MUELLER AND MARKUS MÜLLER, “Timing Recovery in Digital Synchronous Data Receivers”, IEEE TRANSACTIONS ON COMMUNICATIONS, pp. 516-531, VOL. COM-24, NO. 5, MAY 1976.